The present invention relates to a non-reciprocal circuit element for use in high-frequency electrical parts for microwave band.
The non-reciprocal circuit element is utilized in a transmitting circuit of a mobile communication apparatus such as a cellular radiotelephone and an automobile radiotelephone to transmit microwave signal in only one direction. Recent demand for a mobile communication apparatus with a reduced size and a higher performance requires to reduce the size and enhance the performance of the non-reciprocal circuit element itself. To meet such a demand, a non-reciprocal circuit element of concentrated constant type has become widely used because it may be easily reduced in size.
The non-reciprocal circuit element includes an isolator and an circulator, each having a basic structure practically the same as each other. An exploded view of the respective elements constituting an isolator of concentrated constant type is shown in FIG. 5. A resin case 7 is fitted in a lower yoke 6 made of a thin plate of an electrically conductive, ferromagnetic material. A conductor assembly 4, capacitors C1, C2, C3 and a resistor R are mounted in place in the resin case 7, and electrically connected. A magnet 9 for applying DC magnetic field to the conductor assembly 4, a resin mold 5 for keeping the magnet 9 in position, and an upper yoke 8 for forming a magnetic circuit are mounted as shown in FIG. 5. Finally, the upper yoke 8 is connected to the lower yoke 6 to obtain the isolator.
The conductor assembly 4, the most important portion of the isolator, is shown in FIGS. 6(a) and 6(b) in more detail. As seen from FIG. 6(a), the conductor assembly 4 comprises a shield disk 2 being connected to a ground electrode, three strip electrodes 21, 22, 23 radially extending from the shield disk 2 at 120-degree angles, a ferrite disk 1 and insulating sheets 3, 3. The ferrite disk 1 is put on the shield disk 2, and then three strip electrodes 21, 22, 23 are bent as shown in FIG. 6(b). Specifically, each of the strip electrodes 21, 22, 23 is bent inwardly along the contour of the ferrite disk 1 so as to surround the ferrite disk 1. The bent strip electrodes cross each other at the center of the top surface of the ferrite disk 1 while electrically insulated to each other by the insulating sheets 3, 3 disposed between adjacent bent strip electrodes. After mounted into the resin case 7, the ends of the bent strip electrodes 21, 22, 23 are respectively connected to an inlet/outlet port P1, P2 and the resistor R in the resin case 7 through a matching circuit comprising a capacitor, etc. For example, as shown in FIG. 6(b), the bent strip electrode 21 is connected to the port P1 through a capacitor C1, the bent strip electrode 22 is connected to the port P2 through a capacitor C2, and the bent strip electrode 23 is connected to the resistor R through a capacitor C3.
A microwave signal input to the port P1 creates a high-frequency magnetic field around the strip electrode 21. The high-frequency magnetic field is rotated at a predetermined angle by the interaction with the DC magnetic field from the magnet 9 to induce a microwave signal in the strip electrode 22 clockwise adjacent to the strip electrode 21 by an inductive coupling through the ferrite disk 1. The induced microwave signal is transmitted to the port P2. When a reflected wave of the microwave signal being output from the port P2 is input reversely to the port P2, the reversely input signal induces a microwave signal in the strip electrode 23 clockwise adjacent to the strip electrode 22, the induced microwave signal being absorbed by the resistor R. In this manner, the isolator transmits the microwave signals only in one direction. A circulator is obtained by connecting the strip electrode 23 to another input/output port in place of the resistor R.
To meet the demand for an isolator with a reduced size and a reduced thickness, a conventional isolator of 7 mm square and about 3 mm in thickness has been further miniaturized to a size of 5 mm square and about 2 mm in thickness. The effort for reducing the size and thickness and improving the performance of the conductor assembly 4 has been directed to optimizing the material and reducing the size of the main parts such as the ferrite disk 1, shield disk 2 and the strip electrodes 21, 22, 23 because these parts determine the size of the conductor assembly 4. The effort has been further directed to minimizing the variation in the crossing angle between the bent strip electrodes to improve the electrical characteristics.
For example, Japanese Patent Laid-Open No. 8-8612 discloses a non-reciprocal circuit element characterized in that the conductor assembly is constructed by laminating a plurality of insulating films each having thereon a strip electrode so that the strip electrodes cross at a predetermined angle. It is reported that, with such a laminating, the crossed structure of the strip electrodes can be easily obtained and that the variation in the crossing angle between the strip electrodes is minimized to prevent the electrical properties of the non-reciprocal circuit element from being deteriorated.
Japanese Utility Model Laid-Open No. 5-80008 teaches that the miniaturization of the non-reciprocal circuit element requires both the ferrite disk and the strip electrodes to be reduced in their thickness. This prior art document further teaches that thinner strip electrodes are quite difficult to provide a bent structure with a predetermined crossing angle, thereby to reduce the productivity and result in the deterioration of the electrical properties and the product-to-product variation in the electrical properties. To eliminate the problems, the document discloses a non-reciprocal circuit element characterized in that the strip electrodes are bent onto respective insulating sheets having tacky or adhesive nature to fix the strip electrodes at a predetermined crossing angle.
However, it has been still demanded to further improve the electrical properties of the non-reciprocal circuit element and further reduce the product-to-product variation in the electrical properties.